Publisher Summary This chapter reviews that the Late Wisconsinan North American ice sheet complex consisted of three major ice sheets: (1) the Laurentide Ice Sheet, which was centred on the Canadian Shield, but also expanded across the Interior Plains to the west and south, (2) the Cordilleran Ice Sheet, which inundated the western mountain belt between the northernmost co-terminus United States and Beringia, and (3) the Innuitian Ice Sheet, which covered most of the Canadian Arctic Archipelago north of about 7°N latitude. The ice cover over Newfoundland and the Maritime Provinces of Canada is usually referred to as the Appalachian Ice Complex, because ice flowed out from local centres rather than from the Canadian Shield. All of the peripheral ice sheets were confluent at the Last Glacial Maximum (LGM) with the Laurentide Ice Sheet, and the Greenland Ice Sheet was confluent with the Innuitian Ice Sheet. The nucleus of this complex, the Laurentide, comprised three major sectors, the Labrador Sector, the Keewatin Sector, and the Baffin Sector, named for areas of inception mid probable areas of outflow at LGM and located respectively east, west and north of Hudson Bay. The chapter presents revised maps of North American deglaciation at 500-year and finer resolution. These maps represent an updating of a series prepared nearly two decades ago for the INQUA 1987 Congress.

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An outline of North American deglaciation with emphasis on central and northern Canada

Multibeam surveys can provide detailed bathymetry data for the continental slope from which quantitative descriptors of the seabed terrain (e.g., slope) may be obtained. We illustrate the value of these descriptors for benthic habitat mapping, and highlight the advantages of multiscale analysis. We examine the application of these descriptors as predictor variables for species distribution models, which are particularly valuable in the deep sea where opportunities to directly survey the benthic fauna remain limited. Our initial models are encouraging and suggest that wider adoption of these methods may assist the delivery of ecologically relevant information to marine resource managers.

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Multiscale Terrain Analysis of Multibeam Bathymetry Data for Habitat Mapping on the Continental Slope

https://notendur.hi.is/~oi/AG-326%202006%20readings/Canadian%20Arctic/Dyke_QSR2002.pdf

The Laurentide and Innuitian ice sheets during the Last Glacial Maximum

With polar temperatures ∼3–5 °C warmer than today, the last interglacial stage (∼125 kyr ago) serves as a partial analogue for 1–2 °C global warming scenarios. Geological records from several sites indicate that local sea levels during the last interglacial were higher than today, but because local sea levels differ from global sea level, accurately reconstructing past global sea level requires an integrated analysis of globally distributed data sets. Here we present an extensive compilation of local sea level indicators and a statistical approach for estimating global sea level, local sea levels, ice sheet volumes and their associated uncertainties. We find a 95% probability that global sea level peaked at least 6.6 m higher than today during the last interglacial; it is likely (67% probability) to have exceeded 8.0 m but is unlikely (33% probability) to have exceeded 9.4 m. When global sea level was close to its current level (≥-10 m), the millennial average rate of global sea level rise is very likely to have exceeded 5.6 m kyr-1 but is unlikely to have exceeded 9.2 m kyr-1. Our analysis extends previous last interglacial sea level studies by integrating literature observations within a probabilistic framework that accounts for the physics of sea level change. The results highlight the long-term vulnerability of ice sheets to even relatively low levels of sustained global warming. Sea levels at the last interglacial, about 125,000 years ago, were higher and polar temperatures up to 5 °C warmer than today, so the period is seen as a partial analogue for what could happen in the event of anthropogenic warming. Kopp et al. assemble a global database of local sea level rise indicators and use a statistical treatment to estimate global sea level rise during the last interglacial. They find that global sea level was probably 8 to 9.4 metres above present levels, with the rate of sea level rise exceeding 50 cm per century. This suggests that today's ice sheets would be vulnerable to relatively low levels of global warming. Sea levels during the last interglacial stage (about 125 kyr ago) are known to have been higher than today, and may serve as a partial analogue for anthropogenic warming scenarios. However, because local sea levels differ from global sea level, accurately reconstructing past global sea level requires an integrated analysis of globally distributed data sets. An extensive compilation of local sea level indicators and a statistical approach are now used to estimate global sea level during the last interglacial.

Probabilistic assessment of sea level during the last interglacial stage

A review of approaches for classifying benthic habitats and evaluating habitat quality.

This study presents results of a new approach for sea floor habitat mapping based on an integrated analysis of multibeam bathymetric data, associated geoscientific information, and benthos data from Browns Bank on the southwestern Scotian Shelf, off the Canadian Atlantic coast. Based on sea floor sediment maps and statistical analysis of megabenthos determined from photographs, 6 habitats and corresponding associations of benthos were derived and mapped. The habitats are dis- tinguished primarily on the basis of sediment type and water depth. Additional factors are sea floor geomorphology, habitat complexity, and relative current strength. A Browns Bank benthic habitat map is developed as a conceptual model summarizing the understanding of the bank ecology. This study highlights the utility of multibeam bathymetric sonar for interpretation of sea floor sediments and for extrapolating benthic habitat characteristics across large areas of sea floor.

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Benthic habitat mapping on the Scotian Shelf based on multibeam bathymetry, surficial geology and sea floor photographs

A conceptual model of the deglaciation of Atlantic Canada

Seismostratigraphic studies on the Scotian and mid-Norwegian Continental Shelves have led to the development of a conceptual model for glacial deposition and erosion by ice sheets in a marine environment (till-tongue model). Some of the terminal deposits of the marine-ending glaciers are composed in part of acoustically incoherent, wedge-shaped bodies. The wedges are thought to be part of a grounding-line system associated with a floating terminus. According to the model, the sedimentary wedges are formed through the accumulation of glacial debris by subglacial processes proximal to the grounding line, together with a penecontemporaneous, proglacial contribution from sediment gravity flows. Sedimentation rates appear to be highest near and proximal to the grounding line; that is, within a grounding-line zone. Ice-proximal, stratified glaciomarine sediment accumulates from plumes within the water column and rain out of ice-rafted debris. The glaciomarine sediment interfingers with the till tongues, resulting from oscillations of the grounding line. Till tongues represent discrete stratigraphic units formed in both vertical and lateral successions, marginal to massive till deposits. Major till tongues can be correlated regionally for hundreds of kilometers and thus can provide a basis for stratigraphic correlation. On the mid-Norwegian Shelf, lateral and vertical successions of till tongues delineate several large-scale till units which represent major advances and retreats of the ice that probably were of both stadial and glacial rank. We refer to this type of succession as a "till-tongue stratigraphy." It appears that till-tongue formation is limited to intermediate water depths which range from 75 to 1,000 m (present water depth), but those formed in water depths >600-700 in may not be strictly subglacial in origin. We speculate that processes leading to the development of till tongues possibly give rise to the formation of subglacial undermelt till, together with thin flow tills near the grounding line. It is also possible that another type of subglacial till, embracing concepts inherent in the expression "deforming basal tills" as described by Alley and others (1989), may contribute to the formation of till tongues. Boundaries between the major till units of the mid-Norwegian Shelf are mainly depositional in the zone of maximum glaciation near the shelf edge and upper slope. Proximal to this zone of major deposition, intraformational erosional processes dominate and give rise to smooth, widespread, time-transgressive, erosional surfaces. Erosion progressively increases in a proximal direction, beveling the earlier glacial deposits, and ultimately reaches and modifies the bedrock surface. These regional surfaces are analogous to the Ross Sea unconformity which was also formed by glacial erosion. Morainic deposits within a till-tongue stratigraphy are termed "grounding-zone moraines" and are subdivided into advance and retreat varieties, with linear, tabular, and hummocky attributes. Fields of small morainic ridges (lift-off moraines) form in basinal areas during recession.

Till-tongue stratigraphy

The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention. Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current. Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks. Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.

Gas-related sedimentary features from the eastern Canadian continental shelf

Pockmarks, cone-shaped depressions that occur in unconsolidated fine sediments at the seabed, are generally thought to be formed by gas ascending from underlying sediments. A mosaic of pockmarks in Emerald Basin on the Scotian Shelf was constructed and the problems of mosaic construction are discussed. Within the study area pockmarks have an average depth of 6 m, an average diameter of 85 m and cover up to 18% of the seabed. Pockmarks that occur in thin clay (LaHave clay) are generally < 5 m deep and those in thick clay are up to 15 m deep. Many pockmarks are elongate in shape and show a preferred orientation, both of which may be due to the influence of bottom currents. Within the study area, the process of pockmark formation has displaced 2.5 × 105 m3 km−2 of sediment. Pockmarks on the Scotian Shelf appear to be inactive (relict) and in cases where they are buried they are referred to as ancient.

Gordon B. J. Fader

Papers

Benthic habitat mapping on the Scotian Shelf based on multibeam bathymetry, surficial geology and sea floor photographs

A conceptual model of the deglaciation of Atlantic Canada

Till-tongue stratigraphy

Gas-related sedimentary features from the eastern Canadian continental shelf

A side-scan sonar mosaic of pockmarks on the Scotian Shelf